The present invention generally relates to microwave pulse compressors, and more particularly to microwave pulse compressors capable of producing short output pulses (typically nanosecond pulses) from relatively long (typically microsecond) pulse inputs.
Short pulse switched microwave compressors have been designed and fabricated using a fundamental mode rectangular copper waveguide resonator, that is, a length of copper waveguide having a cross-sectional size large enough to propagate and store energy in the fundamental mode, but small enough to exclude higher order modes. This type of pulse compressor stores microwave energy fed into the resonator from a pulse source, typically a magnetron or klystron, over a pulse length of a few microseconds. After a fill time, this stored energy is abruptly “switched-out” as a shorter nanosecond pulse through an output coupled to the waveguide resonator. The resonator guide is long compared to the broad and narrow wall dimensions. An output coupling scheme is devised so that, in theory, limited or zero power is coupled to an output port during the fill time, and then is abruptly and strongly coupled to this port at switch-out.
To illustrate the theory of operation of short pulse switched microwave compressors, consider a rectangular fundamental mode waveguide resonator as having shorting plates at each end of the length of the resonator guide. One of these shorting plates has a small input hole or aperture for coupling a source of input pulse power to the resonator guide. This is the input end of the resonator guide. Both plates act to reflect the traveling wave in the resonator guide resulting from the pulse power introduced at the input end. Introduction of input pulse power at the input end results in a build-up of stored energy in the resonator, which occurs during a “fill time.” (The length of the resonator waveguide must be a multiple of half guide wavelengths to resonate and to allow stored energy to build during the fill time.) Assuming an output waveguide is coupled to the end of the resonator guide opposite the input end, by instantaneously removing the shorting plate (switch-out) at the output end, the energy stored in the resonator guide is released as a traveling wave in the output waveguide. Power traveling toward the output guide at switch-out would first flow into the output waveguide, followed by power that had been reflected back toward the input end at switch-out. This reflected power would travel back toward the shorting plate at the guide's input end and then be reflected back to the output waveguide. The time it takes for this to occur (in nanoseconds) defines output pulse length. The output pulse power is the power level of the traveling waves within the resonator at switch-out. Because the output pulse times are on the order of nanoseconds, the removal of the shorting plate as described above would have to be accomplished in a fraction of a nanosecond. This is not possible, so other switch-out schemes are required.
Instantaneous switch-out has been achieved using a gas plasma switch in front of a shorting end wall or plate at the end of a rectangular resonator guide which is opposite the guide's input end. Using such instantaneous switch-out schemes, power is coupled out through the short sidewall of the resonator guide at a position of maximum or near maximum longitudinal magnetic field when the plasma switch is fired.
A drawback to the above-described short pulse microwave compressors is that, at room temperatures, the fundamental mode waveguide structures used are limited to modest pulse power gains. This is principally due to low unloaded quality factors. For example, a QO of about 10,000 to 12,000 can be expected at 3.0 GHz frequency using a copper waveguide resonator fabricated of a section of a CPR284 waveguide. The power level of the output pulse is constrained by the Q of the resonator guide structure, the power and pulse length (i.e., time) of the drive source, and the input coupling coefficient.
The present invention provides for a short pulse microwave compressor that, for a given input pulse power level, pulse length, and input coupling coefficient, is capable of producing short output pulses at higher power levels than can be achieved by conventional short pulse switched compressors.